Fuel electrode for solid electrolyte fuel cells and a method for manufacture of the electrode

ABSTRACT

The invention relates to a fuel electrode for high-temperature solid electrolyte fuel cells and a process for manufacture of the electrode. The fuel electrode of the invention is a porous element composed of a high-melting metal, such as ruthenium, osmium, rhodium or iridium, or an alloy containing the metal. The process for manufacture of the fuel electrode comprises coating an electrode material with a solution or dispersion of the high-melting metal and/or its chloride, sintering the same and finally reducing the product.

This is a Continuation of application Ser. No. 09/557,511 filed Apr. 25,2000, now U.S. Pat. No. 6,432,570, which is a Continuation of 08/748,120filed Nov. 12, 1996, now Issued U.S. Pat. No. 6,054,232, which is aContinuation of 08/532,317 filed Sep. 20, 1995, now Abandoned, which isa Continuation of 08/340,963 filed Nov. 17, 1994, now Abandoned, whichis a Continuation of 08/195,420 filed Feb. 14, 1994, now Abandoned,which is a Divisional of 07/650,138 filed Feb. 4, 1991, now Issued U.S.Pat. No. 5,286,580.

BACKGROUND OF THE INVENTION

The present invention relates to a fuel electrode for high-temperaturesolid electrolyte fuel cells and a method of manufacturing theelectrode.

Heretofore, nickel/zirconia (Ni/ZrO₂) cermets have been employed forfuel electrodes in commercialized high-temperature solid electrolytefuel cells.

These Ni/ZrO₂ cermets are typically produced by the below alternativeprocesses.

(1) A process in which NiO/ZrO₂ is sintered and then reduced to providea Ni/ZrO₂ cermet, (e.g. the Ceramatec process described in Proceedingsof SOFC-NAGOYA, p.24); and

(2) A process, known as the Westinghouse process, in which ZrO₂ in acrude Ni/ZrO₂ cermet is caused to grow into spaces between Ni grains bythe EVD technique, (Japanese Kokai Patent Publication No. 61-153280).

Aside from the above, porous platinum (Pt) materials obtainable bysintering platinum pastes have been used in ZrO₂ sensors and the like orin basic research.

However, the Ni/ZrO₂ cermet electrode manufactured by thefirst-mentioned process is disadvantageous in that when the fuel cell isrun over thousands of hours at temperatures near 1,000° C., the Nigrains therein are sintered thus degrading the electrode and ultimatelycausing exfoliation of the electrode. Increasing the proportion of ZrO₂to avoid this sintering detracts from the performance of the electrode.

The Ni/ZrO₂ cermet manufactured by the second process is resistant tosintering and insures a satisfactory electrode performance, but, sinceit involves an EVD step, the overall manufacturing process iscomplicated and the cost of manufacture is increased.

The porous platinum electrode is disadvantageous in that the platinumreacts with the impure metal in the fuel and is vaporized in acontinuous operation in a reducing atmosphere resulting in early agingof the electrode. Furthermore, platinum is an expensive metal and themanufacture of porous platinum electrodes is costly.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the abovedisadvantages and provide an improved fuel electrode which does notundergo sintering or aging even when the fuel cell is operatedcontinuously for a long time at a high temperature and a simple methodof manufacturing the electrode.

The fuel electrode of the invention for high-temperature solidelectrolyte fuel cells is characterized in that it is a porous elementcomposed of a high melting point metal having a melting point of notless than 1,900° C. or an alloy containing such a high melting pointmetal.

As species of the high melting point metal with a melting point of notless than 1,900° C., there may be mentioned ruthenium (Ru), osmium (Os),rhodium (Rh) and iridium (Ir).

As apparent from the melting points presented below, Ru, Os, Rh and Irmelt at higher temperatures than Ni and Pt and are more resistant tosintering. Because they are highly resistant to sintering,ceramification is not required. Furthermore, since the resistance tosintering is high, it is feasible to design a system with a longinterfacial dimension of the three-phase zone of ZrO₂/Metal/Gas (if sucha structure is designed with Ni, sintering soon occurs) and therebyreduce all the reaction polarization, diffusion polarization andresistance polarization of the electrode.

Melting Points Ni 1453° C. Pt 1769° C. Ru 2500° C. Rh 1960° C. Ir 2443°C. Os 2700° C.

Of the above-mentioned metals, Ru is the most advantageous in that ithas:

(1) a high melting point and stability even in a reducing atmosphere;

(2) a low cost; and

(3) a high catalyst activity for steam reforming of CH₄ which is animportant factor in electrode performance.

Osmium (Os) is also useful but in view of the high vapor pressure ofOsO₄ and its high toxicity, it is somewhat disadvantageous forcommercial use as compared with Ru.

The fuel electrode of the present invention for high-temperature solidelectrolyte fuel cells can be manufactured by dissolving or dispersing apowder of at least one of Ru, Os, Rh and Ir or at least one of rutheniumchloride, osmium chloride, rhodium chloride and iridium chloride in anorganic solvent or water, coating an electrode material with theresulting solution or dispersion, sintering the same and finallyreducing it. It should be understood that the powdery metal and metalchloride can be used in combination with each other.

The sintering operation is preferably conducted in an oxidizingatmosphere at a temperature of 400 to 1,000° C. and the reductionreaction is preferably conducted at a temperature of 500 to 1,300° C.

Preferably the reduction is conducted in a reducing atmosphere,particularly in a hydrogen (H₂) atmosphere.

The organic solvent to be used is preferably a low-boiling point solventand more desirably an alcohol of 1 to 6 carbon atoms. The mostadvantageous alcohols are propanol and butanol.

In terms of the ease of porosity control and handling, it is goodpractice to employ a high molecular weight compound soluble in theorganic solvent or in water. Among such high molecular weight compoundsare polybutyl alcohol, polyvinyl alcohol and methylcellulose.

Compared with the conventional Ni/ZrO₂ cermet electrode, the electrodeaccording to the present invention has the following meritoriouscharacteristics:

(1) Substantially no sintering occurs even when the fuel cell is run forthousands of hours at 1,000° C.; and

(2) Each of the reaction, diffusion and resistance polarization valuesare as small as several mV at 500 mA/cm².

Particularly when Ru is employed, not only an excellent electrodeperformance is ensured but also the manufacturing cost is kept downresulting in profitable commercial application.

Furthermore, since the manufacturing method according to the presentinvention is a simple process comprising coating an electrode materialwith an aqueous or organic solution or dispersion of a powdery highmelting point metal and/or a high melting point metal chloride,sintering the same, and then reducing it, the manufacturing process issimple and the cost of manufacture is low. Moreover, the concomitant useof a high molecular weight compound soluble in the organic solvent orwater facilitates porosity control and handling.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples and comparative examples are intended toillustrate the invention in further detail and should by no means beconstrued as defining the metes and bounds of the invention.

EXAMPLE 1

In 89 parts by weight of n-butanol were dissolved 10 parts by weight ofhydrous ruthenium trichloride (RuCl₃.3H₂O) and 1 part by weight ofpolybutyl alcohol, and pellets (2 cm in diameter and 0.2 mm thick) ofyttria-doped zirconia (ZrO₂ containing 8 mol % of Y₂O₃; hereinafterreferred to as YSZ) were coated with the resulting solution. The coatedpellets were heated in air at 850° C., and then reduced in a H₂atmosphere at 1,000° C. to provide a porous electrode having a thicknessof 50 μm. Using this as the fuel electrode, a fuel cell was fabricatedand a fuel gas composed of 90 vol. % of H₂ and 10 vol. % of H₂O and airwere passed to the fuel electrode and an air electrode, respectively, toinvestigate the polarization characteristics at 200 mA/cm². As a result,all the reaction, diffusion and resistance polarization values were notlarger than 5 mV. The same fuel electrode was maintained in H₂ streamsat 1,000° C. for 1,315 hours and similar determinations were made. As aresult, no significant difference was found in any of these parametervalues.

EXAMPLE 2

To 30 parts by weight of a 5% aqueous solution of polyvinyl alcohol wereadded 70 parts by weight of powdery ruthenium metal (particle size 1 to5 μm) followed by thorough mixing. Then, YSZ pellets were coated withthe resulting slurry. The coated pellets were heated in the air at 500°C. (for removal of the binder), and then were reduced in streams of agas composed of 5 vol. % of H₂ and 95 vol. % of N₂ at 1,050° C. toprovide a porous electrode having a thickness of 50 μm. The performanceof this electrode was comparable to that of the electrode obtained inExample 1.

EXAMPLE 3

In 89 parts by weight of water were dissolved 10 parts by weight ofhydrous ruthenium trichloride and 1 part by weight of polyvinyl alcohol.YSZ pellets were coated with the resulting solution and, thereafter, theprocedure described in Example 2 was repeated to provide an electrode.The performance of this electrode was comparable to that of theelectrode obtained in Example 1.

EXAMPLE 4

In 35 parts by weight of a 5% aqueous solution of polyvinyl alcohol weredissolved 3 parts by weight of ruthenium chloride followed by theaddition of 62 parts by weight of powdery ruthenium metal to prepare aslurry. YSZ pellets were coated with the above slurry and the coatedpellets were dried at room temperature and heated in the air at 500° C.Then, the pellets were reduced in streams of a gas composed of 5 vol. %of H₂ and 95 vol. % of N₂ at 1,050° C. The characteristics of theresulting electrode which was about 50 μm thick were similar to those ofthe electrode obtained in Example 1.

COMPARATIVE EXAMPLE 1

A 70 weight % suspension of powdery NiO plus powdery YSZ (NiO:YSZ=6:4,w/w) in ethanol was used to coat the same YSZ pellets as used inExample 1. The coated pellets were sintered in the air at 1,300° C. and,then, reduced in a hydrogen stream at 1,000° C. to provide a Ni/ZrO₂cermet electrode. This electrode was tested for various parameters as inExample 1. As a result, the sum of reaction polarization and diffusionpolarization values at 500 mA/cm² was 30-100 mV. When the same electrodewas maintained in a stream of H₂ at 1,000° C. for 1,050 hours, extensiveexfoliation took place.

1. A process for manufacturing a fuel electrode comprising: dissolvingor dispersing at least one member of the group consisting of ruthenium,osmium, rhodium, iridium, ruthenium chloride, osmium chloride, rhodiumchloride, and iridium chloride, in an organic solvent or water; coatingan electrode material with the resulting solution or dispersion;sintering the coated electrode at a temperature of 400 to 1,000° C.; andreducing the sintered coated electrode at a temperature of 500 to 1,300°C.
 2. The process according to claim 1, wherein said organic solvent isan alcohol of 1 to 6 carbon atoms.
 3. The process according to claim 2,further comprising use of a high molecular weight compound soluble inthe alcohol.
 4. The process according to claim 1, further comprising useof a high molecular weight compound soluble in water.
 5. The processaccording to claim 4, wherein said high molecular weight compound ispolyvinyl alcohol.